An engine is provided with an ignition coil having a coil main body constructed by, for example, coaxially arranging a primary coil with a secondary coil. The primary coil is constructed by winding a wire around a primary spool to form a primary winding, and the secondary coil is constructed by winding a wire around a secondary spool to form a secondary winding. A center core, which is formed of a magnetic material, is provided on the radially inner side of the primary and secondary coils. An outer core, which is formed of a magnetic material, is provided on the radially outer side of the primary and secondary coils. Thus, the center core and the outer core construct a magnetic circuit. The ignition coil has an inner gap charged with thermosetting resin such as epoxy resin to electrically insulate inner components from each other, and to fix the inner components therebetween.
The secondary coil has a high voltage end defining a plug mount portion to which a sparkplug is provided. The plug mount portion has a cap mount portion, which is in a cylindrical shape, and extends from a spool constructing the primary coil or the like. The cap mount portion is attached with a plug cap formed of rubber. The plug cap has a fitting hole accommodating a coil spring electrically conducted with a high voltage winding end of the secondary coil via the high voltage terminal. The sparkplug is mounted to the plug mount portion by fitting an insulator portion of the sparkplug into the fitting hole. The sparkplug has a terminal portion in contact with the coil spring.
For example, an ignition coil disclosed in U.S. Pat. No. 6,836,203 B2 (JP-A-2003-163126) has a cap mount portion having an annular charging portion, into which thermosetting resin is charged to restrict electric leakage from a high voltage terminal applied with high voltage to a plug hole in low voltage. This ignition coil disclosed in U.S. Pat. No. 6,836,203 B2 has a coil portion including the primary coil and the secondary coil inserted into a plughole of a cylinder head cover of an engine.
However, the above conventional structure of the ignition coil is not sufficient to restrict electric leakage from the high voltage terminal to the plughole. Specifically, high voltage applied to the high voltage terminal may be transmitted along the inner circumferential periphery of the annular charging portion, and further transmitted toward the radially outer side of the cap mount portion. Accordingly, in the conventional structure of the ignition coil, the insulation distance needs to be sufficiently secured to restrict electric leakage from the high voltage terminal to the plughole.